The Pathogenicity Of Velogenic Viscerotropic Newcastle Disease Virus In The Bursa Of Fabricius

Abstract

Newcastle disease (ND) is one of the important poultry diseases in
Malaysia. Although the disease is controlled through vaccination programmes,
outbreaks have been reported in vaccinated flocks from time to time. The success
of any vaccination programme depends on several factors and one of them is the
integrity of the immune system. The bursa of Fabricius is an important organ of
the immune system which is responsible for humoral immunity against ND.
Realising its importance, a study was undertaken to investigate the effect of
velogenic viscerotropic Newcastle disease virus (VVNDV) on the bursa of
Fabricius. An initial detailed study on the normal structure of the bursa of
Fabricius was conducted as a basis for differentiating the effect of VVNDV on
the bursa of Fabricius.The effect of VVNDV on the bursa of Fabricius of vaccinated chickens
was determined by infecting these chickens via contact with non-vaccinated
chickens challenged intranasally with 0. 1 ml of inoculate containing 106 E1DSO of
the virus. The damage to the bursa of Fabricius was examined by histological and
electron microscopic studies, while virus was detected by isolation,
immunoperoxidase staining and transmission electron microscopy. The results showed that VVNDV damaged and replicated in the bursa of
Fabricius of vaccinated birds. Grossly, the bursa was swollen, oedematous,
haemorrhagic and necrotic. Histopathology showed that there were haemorrhages,
oedema, cystic cavities in the follicles containing mucous, necrosis of the follicles
and presence of a reduced number of lymphocytes and an increased number of
heterophils, macrophages and plasma cells. Scanning electron microscopy showed
increased mucous secretion and there was exfoliation of the microvilli. However
the follicle-associated-epithelium was intact in the vaccinated challenged birds. In
the non-vaccinated challenged birds there was total exfoliation of the epithelium.
Viral isolation and immunoperoxidase staining showed evidence of viral
replication in the bursa of Fabricius. Ultrastructurally, viral replication was shown
by the presence of virus budding and nucleocapsids in the cytoplasm of lymphoid
cells and macrophages.